1. Field of the Invention
The present invention relates to a spread illuminating apparatus for use with a liquid crystal display device, and more specifically to a spread illuminating apparatus equipped with a means for increasing utilization efficiency of light emitted from a light source.
2. Description of the Related Art
A spread illuminating apparatus of side light type, in which at least one light source is disposed at an end surface of a light conductive plate, is heavily used as a subsidiary illuminating apparatus for a liquid crystal display (LCD) device. A bar-like lamp as a light source, which has a length substantially equal to the longitudinal dimension of an end surface of a light conductive plate, is disposed at the end surface in order to illuminate the light conductive plate entirely in a uniform manner.
In an electronic device, such as a portable telephone, which demands downsizing and lower power consumption, a light emitting diode (LED) is used as a light source to meet the demand. However, since the LED is a spot-like lamp, a plurality of LED's have to be disposed at an end surface of a light conductive plate in order to illuminate the light conductive plate entirely in a uniform manner. Meanwhile, FIG. 8 shows a spread illuminating apparatus (refer to Japanese Patent Application Laid-Open No. H10-199316) which is adapted to illuminate a light conductive plate entirely in a uniform manner by means of only one LED. As shown in FIG. 8, a light conductive plate 2 has an optical pattern 2D formed at a part of an end surface 2C thereof. The optical pattern 2D is composed of, for example, a prism array, and an LED 1 is disposed in front of the optical pattern 2D. Light emitted from the LED 1 is scattered at the optical pattern 2D thereby producing lights f1 in addition to lights f0 traveling straightforward. The lights f1 contribute to increasing brightness at the proximal corners of the light conductive plate.
Under the circumstances, in a spread illuminating apparatus for an electronic device, such as a portable telephone, enhanced brightness is also strongly demanded while reduced power consumption is demanded as mentioned above, and therefore it is desired that light emitted from an LED or LED's be used more efficiently. In the spread illuminating apparatus shown in FIG. 8, however, light emitted from the LED 1 is not fully used. Specifically, as shown in FIG. 6A, some of lights PR, which are emitted from an LED 1 in directions forming a large angle with respect to major surfaces of the light conductive plate 2, are not duly introduced into the light conductive plate 2 or even fail to fall incident on an end surface 2C of the light conductive plate 2 because of an open space present between the LED 1 and the light conductive plate 2, and are just wasted. This happens not only in connection with use of an LED but also a fluorescent lamp or a light conductive bar. Referring now to FIG. 6B, a transparent adhesive is filled between the LED1 and the light conductive plate 2 to thereby inhibit reflection loss (coupling loss) conventionally incurred due to the presence of the open space. Here, lights PR have their traveling directions changed to some degree due to the refractive index of the adhesive filled, but not much effectively, so there are still some lights not introduced in the light conductive plate 2, and utilization efficiency of light emitted from the LED 1 remains poor therefore hampering enhancement in brightness as well as reduction in power consumption.
In connection with FIG. 6B, a light converging element, for example, a well-known SELFOC (registered trademark) lens, which has a column shape and has a graded refractive index profile in a radial direction, may be disposed in place of the adhesive at the open space between the LED 1 and the light conductive plate 2. The aforementioned SELFOC lens is formed of a glass base material having a columnar configuration, and is utilized for optical communication, optical measurement instrument, optical information processing, and the like. However, the SELFOC lens focuses light entirely toward its optical axis, and is not adapted to converge light only in a particular direction, for example, vertically as shown by Y in FIGS. 6A and 6B.
A cylindrical lens is another well-known light converging element, which is formed of a glass base material, is shaped in a cylindrical configuration, and which has a uniform refractive index profile. The cylindrical lens has light falling incident on its circumferential surface and can converge light only in a particular direction, for example, vertically in FIGS. 6A and 6B. But, the cylindrical lens has a uniform refractive index profile as described above, and therefore does not provide a good convergence characteristic. Further, since the cylindrical lens has an aced incidence surface so as to converge diffused light in a parallel manner, an open space still remains toward the LED 1 thus generating reflection loss. In order to eliminate the reflection loss, a transparent adhesive or resin may be filled between the LED 1 and the cylindrical lens. This, however, results in that the refractive index of the cylindrical lens is defined as relative to the refractive index of the filled adhesive or resin therefore significantly deteriorating the converging performance of the cylindrical lens. A cylindrical lens having a thickness of 1 to 2 mm and formed of glass having a high refractive index may duly work, but to produce such a cylindrical lens is difficult or prohibitive in view of physical strength, manufacturing technology and production cost. On the other hand, a cylindrical lens formed of synthetic resin does not achieve required performance because the synthetic resin has a lower refractive index than the glass therefore lowering its convergence factor.
Referring to FIG. 7, a light conductive plate 2 is dimensioned to define a major surface having a larger area than a display screen size of an LCD device (not shown) arranged over the light conductive plate 2, on the grounds that a so-called dead area 2A exists inherently toward an end of the light conductive plate 2 provided with LED's 1, where light emission from the LED's 1 is not uniform. In case of a small device, such as a portable telephone, the dead area 2A, which is not usable, has its length Ld usually measuring 2 to 4 mm from the end with the LED's 1. The dead area 2A is covered up to prevent light from exiting out to be wasted and therefore is of no use thus only hampering downsizing of the device.